The present invention relates generally to improvements in the electrodeposition of zinc and zinc alloys from aqueous alkaline plating baths and to new additives for use in such electrodeposition processes.
Electrodeposition of zinc and zinc alloys, based for example on sodium zincate, has been known for many years. It is not possible to produce a commercially acceptable deposit from a simple sodium zincate electrolyte as the deposit is powdery and dendritic. For this reason, various additives have been proposed to provide improved deposition, such as cyanides (which have obvious environmental problems) and polymers of amines and epichlorohydrin which act as grain refining additives. These polymers are limited to usage in baths having relatively low concentrations of zinc because it is not possible to prevent uncontrolled deposition of zinc at higher metal concentrations. Also, electroplating processes using these additives tend to have poor cathode efficiency, a narrow bright range, a narrow operating window and tend to produce pitted and xe2x80x9cburntxe2x80x9d deposits. More recently, additives have been proposed which allow higher zinc concentrations to be used, which have significantly reduced burning and pitting and which allow a wider range of operating parameters. Further, the additives enable an excellent deposit distribution (that is, evenness of the deposit across the article being plated, irrespective of its shape in particular areas). This maximises the efficiency of zinc usage. These additives are based generally on polyquaternary amine compounds and are described in U.S. Pat. No. 5,435,898 and U.S. Pat. No. 5,405,523, which also provide further discussion of the prior art.
U.S. Pat. No. 5,435,898 describes polymers for use as additives in the electrodeposition of zinc and zinc alloys, the polymers having the general formula: 
R1 to R4 may be the same or different and are, inter alia, methyl, ethyl or isopropyl and Y may be S or O. R5 is an ether linkage such as (CH2)2xe2x80x94Oxe2x80x94(CH2)2.
U.S. Pat. No. 5,405,523 claims ureylene quaternary ammonium polymers in general as brightening agents in zinc alloy electroplating baths. The preferred and exemplified polymers include units of the general formula: 
where A may be O, S or N and R may be, inter alia, methyl, ethyl or isopropyl. In the preferred polymers, these units are linked by units derived from, for example a bis(2-haloethyl) ether, a (halomethyl) oxirane or a 2, 2xe2x80x2-(ethyleredioxy)-diethylhalide. Ethylene dihalides such as ethylene dichloride and ethylene dibromide are also suggested but not exemplified.
Further known additives are polycationic compositions based on polymerisation of dimethyl-diallyl ammonium chloride with sulphur dioxide as described in DE 19,509,713.
However, the overall cathodic efficiency of these processes can be low and the resultant deposits may be unsatisfactory in terms of brightness and levelling.
The present invention provides improved polymers for use as additives in the electrodeposition of zinc and zinc alloys. In particular, it has been found that by avoiding an ether-type linkage such as R5 in the prior art above, a brighter deposit may be obtained which is also easier subsequently to apply conversion coatings.
The present invention is thus concerned with electrodeposition on a variety of electrically conducting substrates in a medium which may provide improved cathode efficiency and/or improved brightness and/or a more stable finish which is suitable for further treatment. Suitable substrates include iron and all ferrous-based substrates (including both iron alloys and steels), aluminium and its alloys, magnesium and its alloys, copper and its alloys, nickel and its alloys, and zinc and its alloys. Aluminium and its alloys and ferrous-based substrates are particularly preferred substrates, with steels being most preferred.
In its broadest sense, the invention provides polymers for use as additives in the electrodepostion of zinc and zinc alloys, and processes employing the polymers, the polymers being obtained by the reaction of one or both of:
(a) a di-tertiary amine containing an amide functional group and
(b) a di-tertiary amine containing an alkyl group, with
(c) a di-halo alkane, to form a random co-polymer.
The present invention also relates to a method of coating an electrically conducting substrates with zinc or zinc alloy by electrodeposition from a bath medium comprising of an effective amount of the reaction product of one or both of: (a) di-tertiary amine containing an amide functional group and (b) a di-tertiary amine containing an alkyl group, with (c) a di-halo alkane, to form a random co-polymer, a source of zinc ions and optionally additional metal ions of one of more alloying metals, and a chelating agent to render the ions soluble.
The di-tertiary amine (a) containing an amide functional group in the polymer of the invention has the general formula: 
where Rxe2x80x2 represents 
and q is 2 to 6,
R is CH3 or C2H5 and each R may be the same or different, and
m is 2 to 4.
An example of a suitable ditertiary amine of Formula (1) is N,Nxe2x80x2-bis[3-(dimethylamino)propyl]urea.
The ditertiary amine (b) containing an alkyl group has the general formula: 
where B is CgH2g+1 and g=0 or an integer the respective B groups being the same or different, and f=0 o an integer, and
Rxe2x80x3 is CH3 or C2H5 and each Rxe2x80x3 may be the same or different. Thus, the amine groups may be terminal or branched with respect to the alkyl chain portion. Preferably, however, the amine groups are terminal, as indicated by the general formula: 
where Rxe2x80x3 is CH3 or C2H5 and each Rxe2x80x3 may be the same or different, and p is at least 2.
Examples of suitable di-tertiary amines of Formula (2) include N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-1,6-hexanediamine, N,N,Nxe2x80x2Nxe2x80x2-tetramethyl-1,3-propane diamine and N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-1,3 butane diamine.
The dihaloalkane (c) may be represented by the general formula:
Axe2x80x94(CH2)nxe2x80x94Axe2x80x83xe2x80x83(4)
where A represents a halogen atom, especially chlorine or bromine and most preferably chlorine, and n is at least 2, provided that if the monomer of formulas (2) or (3) above is absent, n is at least 3.
Examples of the dihaloalkanes of formula (4) include 1,4-dichlorobutane, 1,5-dichloropentane, 1,6-dichlorohexane and 1,3-dichlorobutane. The latter is believed to result in a polymer additive which is less effective than those dihaloalkanes where the halogen atoms are in terminal positions only.
The upper limit of n (formula (4)) p (formula (3)) or f and g (formula (2)) respectively is determined by the the need for the resultant polymer to be soluble in the electroplating bath. In practical terms, it is envisaged that the upper limit of n and p respectively will be about 8, that f will be not more than 6 and that g will not be more than 3 as higher values produce polymers of insufficient solubility.
The resultant polymer additive according to the present invention may be represented by the formula: 
Where: 0xe2x89xa6xxe2x89xa61
0xe2x89xa6yxe2x89xa61
and either (x or y) or (x and y)=1
z is at least 2 and when y=0, n is at least 3.
In practice, it may be difficult to produce polymers where n and p both have a value of 2 and also x is 0. For this reason, when x=0, it is preferred that the sum n+p is at least 6.
In the polymer additive of the invention the di-tertiary amine unit containing an amide functional group may be absent (i.e. when x=0) or the di-tertiary amine unit containing an alkyl group may be absent (i.e. y=0), but one or other of these units must be present. Preferably, both units are present. The polymer of the invention when both the above mentioned units are present is a random co-polymer such that the respective di-tertiary amine units appear in random sequence (in all cases linked by the di-halo alkane residue).
The absolute value of z is not specified as the polymer of the invention will normally comprise polymer molecules of a range of molecular weights. For individual polymer molecules z will generally be at least 4 to 20 and may be as high as 100 or more.
Also, the molar ratio in the polymer of the di-tertiary amine units derived from formulas (1) and (2) respectively may be selected as desired in order to achieve particular properties. Thus, a polymer in which y=0 results in a zinc electrodeposition process producing a very bright deposit with good distribution (even coating) but the cathode efficiency is not as high as may be desirable. A polymer where both x and y are greater than 0 provides good brightness and good distribution, together with good cathode efficiency. Preferably, the molar ratio of the di-tertiary amines derived from formulae (1) and (2) is in the range of 25:75 to 75:25. More preferably, the ratio is 50:50 to 75:25, and most especially 62.5:37.5.
For the di-tertiary amine of formula (1), Rxe2x80x2 is preferably 
but when Rxe2x80x2 is 
q is preferably 4 to 6. Further R (irrespective of Rxe2x80x2) is particularly preferably CH3.
In the di-tertiary amine represented by formula (2) Rxe2x80x3 is preferably CH3 and L is preferably 2 to 4 so that in formula (3), p is preferably 4 to 6.
For the dihaloalkane of formula (4), n is preferably in the range of 4 to 6.
The following examples are illustrative of preparation techniques for polymers according to the invention.